Recently, in a medical field, diagnosis has been conducted by means of an insertion-type endoscope having a long insertion portion provided with an imaging device at its front end, and a capsule endoscope in which an imaging device is accommodated in a capsule. The capsule endoscope is formed so as to have a size swallowable for a subject under inspection. Therefore, the capsule endoscope has an advantage in that it can remove not only the load on a patient at the time of swallowing the insertion portion of the insertion-type endoscope but also the load on a patient while the insertion portion of the insertion-type endoscope is kept being inserted in a body cavity of the patient for many hours.
The capsule endoscope is provided with a dome-shaped transparent cover at its front end so as to readily advance along a tubular channel after being swallowed into the body cavity, and a cylindrical capsule main body is connected to the transparent cover. An optical axis of an image pickup optical system is generally designed to pass through a center of the transparent cover. Accordingly, the image pickup optical system receives not only light flux near the optical axis but also light flux made incident thereon with a large incident angle through a peripheral portion of the transparent cover. Further, an object distance tends to be longer on the optical axis, and tends to be shorter as an angle of view in imaging becomes wider. Therefore, in the general image pickup optical system in which an image of a planar object is formed on a planar image pickup surface vertical to the optical axis, a range in which preferable image-forming can be achieved is extremely limited.
Under the circumstances described above, an image pickup optical system having a wide angle of view is known as is disclosed in Patent Document 1. However, if a peripheral portion of the object is made to be in focus in the optical design, a central portion of the object falls outside of the depth of field to be out of focus. In contrast, if the central portion of the object is made to be in focus excessively, the peripheral portion of the object falls outside of the depth of field to be out of focus. As the countermeasure of such a problem, according to an image pickup optical system disclosed in Patent Documents 2 and 3, the image surface is made to coincide with the neighborhood of the image pickup surface in the center of a screen at the maximum angle of view, such that the entire object including not only its central portion but also its peripheral portion falls within the depth of field.